Powder coating compositions

Abstract

The invention relates to a powder coating composition comprising a first powder coating component dry blended with a silica, in which the first powder coating component comprises a curable resin and a curing additive for curing the curable resin and has a Dv90 particle size of 150 pm or less, wherein at least 95 wt% of the dry blended silica is porous silica having a D[4, 3] average particle size of 2 to 20 pm and a pore volume of 0.8 to 3.0 mL / g, and wherein the powder coating composition comprises 0.01 to 0.70 wt% of the porous silica.

Description

[0001] POWDER COATING COMPOSITIONS

[0002] Field of the Invention

[0003] The invention relates to a powder coating composition comprising dry blended components. The invention also relates to a substrate coated with such a powder coating composition, and also a method for coating a substrate with such a powder coating composition.

[0004] Background to the Invention

[0005] Powder coating compositions are solid compositions that generally comprise a solid filmforming (binder) polymer or mixtures of different solid film-forming polymers. The compositions can also comprise other components, for example pigments, extenders and one or more performance additives such as plasticizers, stabilizers, degassing agents, and flow aids. The film-forming polymers are usually thermosetting polymers that cure upon heating, typically in the presence of a crosslinking agent which may itself be a polymer. Generally, the polymers have a glass transition temperature (Tg), softening point or melting point above 30 °C.

[0006] Conventionally, the manufacture of a powder coating composition comprises melt-mixing the components of the composition. Melt-mixing involves high speed, high intensity mixing of dry ingredients followed by heating of the mixture to a temperature above the softening temperature of the uncured polymer, but below the curing temperature, in a continuous compounder such as a single or twin-screw extruder to form a molten mixture. The extruded molten mixture is rolled into the shape of a sheet, cooled to solidify the mixture, crushed into flakes, and then pulverized to a fine powder. Generally, the powder is then subjected to a sequence of particle sizing and separation operations, such as grinding, classifying, sifting, screening, cyclone separation, sieving and filtering.

[0007] The thus-obtained powder coating composition is then applied to a substrate and heated to melt and fuse the particles and to cure the coating. Powder coating compositions may be applied by fluidized-bed processes wherein the substrate is preheated and dipped in a fluidized bed of the powder resulting in the powder fusing on contact with hot surface and adhering to the substrate, by electrostatic fluidized-bed processes, or by electrostatic spray processes wherein the powder coating particles are electrostatically charged by electrodes within a fluidized bed or by an electrostatic spray gun and directed onto an earthed substrate.

[0008] Powder coating compositions are generally formulated as so-called one-component compositions prepared by melt-mixing the binder, crosslinking and additive components together, which can then be extruded and ground or pulverised to form appropriately sized powder coating particles. Optionally, additional powder coating additives can be dry-blended with the extruded particles, for example to improve flowability.

[0009] WO00 / 01774, WO2021 / 245043, WO2021 / 245044, WO2021 / 245045 and WO2021 / 245046 disclose examples of single component powder coating compositions having dry-blended additive components.

[0010] In the production of powder coatings and in their processing, fines can be generated. During spray application, the fines tend not to end up on the substrate but instead begin to accumulate in the spray booth and recycled powder. Therefore, they are typically screened out, either before or during use, which leads to reduced product utilisation. It would be advantageous if the fines could somehow be used or re-used to avoid such waste.

[0011] Summary of the Invention

[0012] It has now been found that a particular dry-blended silica additive can help improve fines utilization.

[0013] The invention provides a powder coating composition comprising a first powder coating component dry blended with silica, in which; the first powder coating component comprises a curable resin and a curing additive for curing the curable resin and has a Dv90 particle size of 150 pm or less, and at least 95wt% of the dry blended silica is porous silica with an average particle size (D[4,3]) in the range of from 2 to 20 pm and a pore volume in the range of from 0.8 to 3.0 mL / g, and the amount of the dry blended porous silica in the powder coating composition is in the range of from 0.01 to 0.70 wt%.

[0014] In a second aspect, the invention provides a method for coating a substrate with such a powder coating composition.

[0015] In a third aspect, the invention provides a substrate coated with such a powder coating composition.

[0016] Brief Description of the Drawings

[0017] Figure 1 is a graph showing the particle size number distribution of the powder coating composition of Comparative Example 3 before spraying and after deposition on a substrate.

[0018] Figure 2 is a graph showing the particle size number distribution of the powder coating composition of Example 3 before spraying and after deposition on a substrate.

[0019] Figure 3 is a graph showing the particle size number distribution of the powder coating composition of Comparative Example 6 before spraying and after deposition on a substrate.

[0020] Figure 4 is a graph showing the particle size number distribution of the powder coating composition of Example 8 before spraying and after deposition on a substrate.

[0021] Figure 5 is a graph showing the particle size number distribution of the powder coating composition of Example 9 before spraying and after deposition on a substrate.

[0022] Figure 6 is a graph of reflectance versus wavelength for powder coating compositions of Comparative Example 3 over various film thicknesses.

[0023] Figure 7 is a graph of reflectance versus wavelength for powder coating compositions of Example 3 over various film thicknesses. Figure 8 shows photographs of the convex (Figure 8a) and concave (Figure 8b) surface of a curved sheet of aluminium, where powder coating of Comparative Example 5 has been applied to the convex surface. The coating in these photographs is not cured.

[0024] Figure 9 shows photographs of the convex (Figure 9a) and concave (Figure 9b) surface of a curved sheet of aluminium, where powder coating of Example 5 has been applied to the convex surface. The coating in these photographs is not cured.

[0025] Figure 10 is a photograph of a recessed aluminium substrate that has a flat upper surface and four recessed areas of different depths. The powder coating composition of Comparative Example 5 has been spray applied to the substrate, the spray being directed into the recessed area from a perpendicular position. The coating in this photograph is not cured.

[0026] Figure 11 is a photograph of a recessed aluminium substrate that has a flat upper surface and four recessed areas of different depths. The powder coating composition of Example 5 has been spray applied to the substrate from directly above and into the recessed area. The coating in this photograph is not cured.

[0027] Figure 12 is a photograph of a side-view of the recessed substrate shown in Figure 10.

[0028] Figure 13 is a photograph of a side-view of the recessed substrate shown in Figure 11 .

[0029] Detailed Description of the Invention

[0030] [General Comments]

[0031] Various particle sizes are mentioned below. Dv90 is the particle size value at which 90% of the total volume of particles has a particle size below that value. Dv50 is the particle size value at which 50% of the total volume of particles has a particle size below that value. Average particle size is the volume-based average or mean particle size, and is often represented as D[4,3], The D in all these measurements represents diameter. Particle sizes, including Dv90, Dv50 and D[4,3] values, can be measured by laser diffraction. One method is ISO 13320 (2020), which in embodiments employs the Mie model.

[0032] Pore characteristics such as pore volumes are conveniently measured by gas adsorption techniques, e.g. nitrogen adsorption. An example method is ISO 15901-2 (2022).

[0033] Numerical particles size distributions, from which the weight% of various size ranges can be calculated (based on density), can be determined by using electrical sensing zone methods (e.g. using a Coulter counter). A suitable method to use is ISO13319 (2021).

[0034] The term “dry blended” components is used to describe physical mixtures of powdered components of the powder coating composition. A dry blend of powder coating components is distinct, for example, from a co-extruded mixture of components, where several components have been mixed and co-extruded together (e.g. by melt extrusion) before the extruded mixture is crushed into powder particles. A dry blend is also distinct, for example, from bonded powder particles where one component is embedded within or bonded with particles of another component, an example being the “bonding” of metallic effect pigment particles with curable resin-containing particles, where the curable resin-containing particles are heated to above the resin’s glass transition temperature, contacted with the metallic effect pigment particles, and then allowed to cool.

[0035] Gloss values and distinctness of image data are commonly measured characteristics of coatings. A method that can be used for measuring gloss is ASTM D523-14 (2018). Distinctness of image measurements can be obtained using ASTM D5767-18 (2023).

[0036] [The powder coating composition]

[0037] The powder coating composition comprises a first powder coating component, and optionally one or more dry blended additional powder coating components. Each powder coating component comprises a curable resin and a curing additive for curing the curable resin.

[0038] Each powder coating component is a powdered material, where the powder particles each comprise both a curable resin and a curing additive. Such powder coating components are sometimes referred to below generally as curable resin-containing components.

[0039] The powder coating composition also comprises a dry-blended silica, at least 95 wt% of which is porous silica having the properties described further below. The silica is a powdered silica.

[0040] In embodiments, the powder coating composition can also comprise a relatively high proportion of small particles (fines) of inter alia the first and any additional curable resin-containing powder coating components.

[0041] [First powder coating component]

[0042] Powder particles of the first powder coating component can be obtained by melt-mixing the constituent ingredients in a compounder such as an extruder. When melt-mixing is employed, the powder particles of the first powder coating component comprise a polymer that is able to soften or melt in the compounder. This polymer can be the curable resin, any polymeric curing additives, or both. Further powder coating ingredients can optionally also be included, such as pigment, extender, or performance additive(s), for example melt flow agent, degassing agent, or dispersing agent.

[0043] Typical Dv90 particle sizes of the first powder coating component are 150 pm or less, for example 130 pm or less. Typically, the Dv90 is at least 20 pm, for example at least 30 pm.

[0044] The first powder coating component comprises one or more curable resins. It also comprises one or more curing additives for the one or more curable resins.

[0045] Collectively, the curable resin(s) and curing additive(s) can be referred to as the curing system. In embodiments, the amount of curable resin and curing additive in the first powder coating component is at least 40 wt%, for example from 40 to 90 wt%, such as from 50 to 80 wt%.

[0046] [Additional powder coating components]

[0047] In embodiments, there can be additional powder coating components comprising one or more curable resins and one or more curing additives for the curable resin(s). These are generally as described above for the first powder coating component, but are compositionally different from the first powder coating component in that at least one of the curable resin or curing additive is different from the first powder coating component, and / or the relative ratio of curable resin to curing additive is different.

[0048] By compositionally different is meant that there is some compositional difference compared to the first powder coating component in terms of the curable resin or the curing additive or both. For example, it could mean that different relative amounts of curable resin and curing additive are used, or that a different curable resin or curing additive is used. Different resins can be either different types of resin (e.g. epoxy versus polyester), or the same type of resin but with different characteristics (e.g. in terms of molecular weight, hydroxyl number, glass transition temperature etc.). Similarly, different curing additives can mean different types of curing additive (e.g. - hydroxyalkylamide versus isocyanate) or curing additives with different characteristics (e.g. different functional group content).

[0049] Any additional powder coating components that may be present are dry blended with the first powder coating component and the dry blended porous silica. In embodiments, there are no more than 50 wt% of such additional powder coating components in the powder coating composition, for example no more than 40 wt%, or no more than 30 wt%. In embodiments, there are no additional powder coating components, the powder coating composition being a so-called “single component” powder coating composition.

[0050] In embodiments, the total amount of first and any additional powder coating components in the powder coating composition is at least 50 wt%, for example in the range of from 50 to 99.99 wt%, for example in the range of from 60 to 99.95 wt% or from 60 to 99.90 wt%.

[0051] [Curing system]

[0052] The term “curing system” can be used to describe the combination of a curable resin and a curing additive within a powder coating component.

[0053] The curing additive is a compound that enables the curing of the curable resin, such as a curing agent, a curing catalyst, a free-radical initiator such as a thermal radical initiator or photo initiator, an accelerator, or an inhibitor. Reference herein to a curing catalyst is to a compound that catalyzes the cross-linking reaction between the curable resin and a curing additive or, in case of a self-crosslinking curable resin, catalyzes the self-crosslinking reaction.

[0054] There can be one or more than one curing additive in the curing system of the first powder coating component and / or any additional powder coating components. The one or more curing additives for curing the curable resin preferably comprise a curing agent that crosslinks with the curable resin and / or a curing catalyst. A crosslinking curing agent may be a resin itself, such as an epoxy resin that crosslinks with a carboxyl-functional polyester resin, or a polyamine resin that crosslinks with an epoxy resin. It will be appreciated that in case of a curing system with a curable resin and a curing agent that is a resin itself, any of the two resins can be considered the curable resin or the curing additive.

[0055] There can be one or more than one curable resin in the first powder coating component and / or in any additional powder coating components. Suitable curable resins are, for example, carboxyl-functional resins such as carboxyl-functional polyesters, polyesteramide or (meth)acrylate-based resins; amine-functional resins such as polyamide or polyester-amide resins; hydroxyl-functional resins such as hydroxy-functional polyesters; epoxy-functional resins (including glycidyl-functional resins); anhydride- functional resins; and resins with unsaturated bonds such as unsaturated polyesters. Curing additives such as crosslinking curing agents or curing catalysts for curing such curable resins are well known in the art. Suitable curing additives for curing carboxyl- functional resins are for example p-hydroxyalkylamides, polyisocyanates such as triglycidyl isocyanurate, and epoxy-functional resins.

[0056] Suitable curing additives for epoxy-functional resins, for example, include amines and isocyanates such as polyisocyanates.

[0057] In one embodiment, the curable resin is a carboxyl-functional polyester, carboxyl- functional polyacrylate, hydroxy-functional polyester or hydroxy-functional polyacrylate. In embodiments, it is a carboxyl- or hydroxyl-functional polyester. Any of these can be used in combination with one or more curing additives selected from a p- hydroxyalkylamide (HAA), a polyisocyanate such as triglycidyl isocyanurate (TGIC), and an epoxy-functional resin.

[0058] In another embodiment, the curable resin (or at least one of the curable resins) is an epoxy-functional resin, and the curing additive (or at least one of the curing additives) is an amine compound or an amine-functional resin.

[0059] In embodiments, the curing system may suitably be an epoxy-polyester system or a polyester-HAA system. In an epoxy-polyester curing system, the curing additive can be an epoxy resin and the curable resin can be a polyester resin with crosslinkable functional groups (such as a hydroxyl-functional polyester of a carboxyl-functional polyester), where the epoxy resin crosslinks with the functional groups on the polyester resin. In a polyester-HAA curing system, the curing system comprises a polyester resin and an HAA.

[0060] Polyester resins that can be used include saturated carboxyl-functional polyester resins. In embodiments, they have an acid value in the range of from 10-100 mg KOH / g, for example in the range of from 20-90, 20-70 mg KOH / g. In embodiments, the glass transition temperature is in the range of from 20-80 °C, for example in the range of from 40-70 or 55-65 °C. In embodiments, the viscosity is 500-7000 mPa.s @200 °C, for example from 1000-5500 or from 1500-4000 mPa.s @200 °C. Epoxy resins can be bisphenol-based epoxy resins. In embodiments, they have epoxy equivalent weights (EEW) of 300-1400 g / eq, for example 400-1200 or 500-900. In embodiments, the softening point is 60 to 120 °C, for example 70 to 115 °C or 80 to 105 °C. In embodiments, their melt viscosity can be in the range of from 300-30000 mPa.s @150°C, for example 600-20000, 800-10000, 1000-7000 or 1200-4000 mPa.s @150°C.

[0061] In embodiments, the curing system is capable of curing at a temperature in the range of from 130 to 240 °C, for example from 140 to 220 °C.

[0062] [Other additives]

[0063] Separate to the dry blended silica (discussed further below), the powder coating composition can comprise one or more other additives, which can be selected from stabilisers, levelling agents, anti-settling agents, matting agents, rheology modifiers, anticorrosion agents, flexibility agents, surface-active agents, UV light absorbers, light stabilisers, amine synergists, waxes, adhesion promoters, fluidisation agents, gloss enhancers, fillers, pigments, flow control agents, degassing agents, and antioxidants.

[0064] The other additives can be included as part of any curable resin-containing powder coating components, e.g. being added before or during a melt-mixing / extrusion process as described above. Additionally or alternatively, they can be bonded to any curable resin-containing powder coating components. Bonding of additive particles (e.g. pigment particles) to curable resin-containing powder component particles is known in the art, and is typically achieved by heating the powder coating component to a heating temperature around the glass transition temperature but below the curing temperature of any curable resin in the powder coating component, under an inert atmosphere; adding the other additive (such as a solid pigment) to the heated powder coating component under stirring whilst maintaining the heating temperature until the additive is bonded to the powder coating component particles, typically after 10-20 minutes; and cooling the powder coating composition.

[0065] Additionally or alternatively, they can be separately dry blended with the curable resincontaining powder coating components and the dry blended porous silica. The total quantities of these other additives can be in the range of from 0 to 40 wt%, for example from 0 to 35 wt%, or from 0 to 30 wt%. Where any such other additives are present, their minimum concentration (individually or cumulatively) is typically at least 0.05 wt% in the powder coating composition, for example at least 0.1 wt%.

[0066] In embodiments, the one or more other additives comprise one or more dry blended inorganic particulate additives. Examples include inorganic colour pigments, inorganic effect pigments such as metal effect pigments, biocidal pigments, anticorrosive pigments, extenders, opacifying pigments, fluidization agents, conductive or anti-static pigments, infrared-absorbing pigments, radiation shielding pigments, glass flakes, abrasion resistance agents or any combination of two or more thereof.

[0067] The total amount of the dry-blended inorganic particulate additives can be up to 35wt%, for example up to 30 wt% or up to 25 wt%.

[0068] In embodiments, the one or more dry blended inorganic particulate additives include dry-blended alumina, aluminium hydroxide, or mixtures thereof. In embodiments, the total amount of dry blended silica, alumina and aluminium hydroxide is in the range of from 0.1 to 1 .5 wt%, for example from 0.1 to 1 .2 wt% or 0.1 to 1.0 wt%.

[0069] [Porous silica]

[0070] The powder coating composition comprises dry blended silica, at least 95wt% of which is porous silica having an average particle size of from 2 to 20 pm as determined by laser light scattering. The content of the dry blended porous silica is in the range of from 0.01 to 0.70 wt%, based on the powder coating composition as a whole.

[0071] Other sources of silica can be present in the coating composition (either as a dry blended additive or incorporated into the first or additional powder coating components).

[0072] The amount of any other dry blended silica in the coating composition (i.e. silica that is not porous, and / or which has a different average particle size) is no more than 5 wt% of the total dry blended silica, for example no more than 3 wt% or no more than 1 wt%. In embodiments, there is no other dry blended silica in the powder coating composition, i.e. 100 wt% of dry blended silica is the porous silica with average particle size of 2 to 20 pm and pore size of 0.8 to 3.0 mL / g.

[0073] In embodiments, there is no more than 5 wt% of any other silica in total in the coating composition based on all silica present in the coating composition (e.g. the total of all dry blended silica and all silica that is incorporated into or bonded with the first or additional powder coating components), for example no more than 3 wt% or no more than 1 wt%. In embodiments, there is no other silica in the powder coating composition.

[0074] By silica is meant a material that can be generally represented by the formula SiC>2. It excludes silicates such as aluminosilicates and silicate salts, and also excludes glasses and ceramics that are made up of oxide mixtures. In embodiments, the porous silica has a bulk density of 0.8 g / cm3or less, for example in the range of from 0.05 to 0.80 g I cm3.

[0075] The porous silica is typically not a fumed silica (i.e. produced by pyrolysis) since these tend to be non-porous. In embodiments, the porous silica is derived from precipitation of silicate solutions (e.g. alkali metal silicate such as sodium silicate) or colloidal silica. The porous silica has a pore volume in the range of from 0.8 to 3.0 mL / g, for example from 1.0 to 2.5 mL / g.

[0076] The porous silica is typically an untreated or unmodified silica, although in embodiments it can also be selected from hydrophilic surface-modified silicas and hydrophobic surface-modified silicas. In further embodiments, the dry-blended silica comprises no polymer-coated or wax-coated silica.

[0077] The use of the dry blended porous silica described herein helps to improve the uniformity of deposition of powder coating particles across a wide range of particle sizes onto the substrate. It also allows an increased proportion of fines to be incorporated into the deposited layer. [Fines]

[0078] Powder coating fines, in particular fines of curable resin-containing powder coating components, are generally undesirable because proportionally (compared to larger particles) they are less likely to form part of the coating layer, and instead tend to accumulate in the recycled powder. Therefore, they typically have to be removed, either before use or during use, resulting in wastage of material. They can also reduce the fluidity of the powder, which makes it difficult to spray.

[0079] In the powder coating compositions described herein, the incorporation of fines in the applied coating layer is significantly improved. This means, for example, that less screening of fines needs to take place before or during the coating application procedure. It also provides an option to enhance the fines content, e.g. by adding fines of the first and / or additional powder coating components to a powder coating composition. It has been found that higher fines content can confer particular advantages in the cured coating, for example high gloss at low coating thickness and improved distinctness of image, particularly at low film thicknesses. In embodiments, at least 85% (on a numerical basis) of fines in the powder coating composition are deposited on the substrate.

[0080] By fines is meant particles having a particle size of 10 pm or less.

[0081] It is possible for the powder coating composition to comprise 5 wt% or more of fines. In embodiments, the amount is 7 wt% or more, 10 wt% or more or 12 wt% or more based on the powder coating composition as a whole. Typically, the total quantity of fines is no more than 50 wt% of the powder coating composition, for example no more than 40 wt% or no more than 35 wt%. Example ranges include 5 to 50 wt%, 5 to 40 wt%, 5 to 35 wt%, 7 to 50 wt%, 7 to 40 wt%, 7 to 35 wt%, 10 to 50 wt%, 10 to 40 wt% and 10 to 35 wt%. Fines content can be determined from the particle number distribution.

[0082] In embodiments, the percentage of fines deposited on the substrate after spray application, compared to the fines in the pre-sprayed coating, is at least 85% on a numerical basis (e.g. as determined by laser diffraction). [Coating application]

[0083] The substrate may be any substrate suitable for powder coating, for example metal, wood, plastic, or substrates comprising any of these materials. In embodiments, the substrate is a metal substrate.

[0084] Prior to applying the powder coating composition, the substrate surface may be treated to remove any contaminants and / or to improve corrosion resistance of the substrate. Such surface treatments are well known in the art and commonly applied to surfaces to be coated with powder coatings.

[0085] The powder coating composition according to the invention may be applied as a topcoat over a first layer of powder coating composition. The first layer can be a powder coating composition different to those described herein. Thus, in one embodiment, the substrate is coated with a first layer of a first powder coating composition, followed by a second layer of powder coating composition according to the present invention.

[0086] The powder coating composition can be applied by spray application, preferably spray application with a corona gun.

[0087] An advantage of the present invention is that high gloss and distinction of image (DOI) can be achieved, particularly at relatively low film thicknesses. Often in polyester-based powder coatings, a relatively thick coating is needed to get sufficient and homogeneous gloss characteristics, e.g. at cured film thicknesses of greater than 50 pm.

[0088] In the present application, high and homogeneous gloss can be achieved at cured film thicknesses of 50 pm or less, for example 45 pm or less, 40 pm or less or even 35 pm or less. The cured coating thickness in embodiments will also be at least 20 pm or at least 25 pm.

[0089] The cured coating can have high gloss characteristics. In embodiments, the cured coating has a 60° gloss value of 85 or more, for example 90 or more. In embodiments, the 60° gloss does not exceed 200, and in other embodiments it does not exceed 150 or 100. Another advantage is good flow and levelling characteristics, which can manifest itself in high distinctness of image values of the cured coatings. In embodiments, the distinctness of image, measured at 60°, is at least 30, for example at least 35. Typically, it does not exceed 200 and in other embodiments it does not exceed 150 or 100.

[0090] The invention will now be illustrated by the following non-limiting examples and the drawings.

[0091] Examples

[0092] [Powder coating examples]

[0093] Compositions of Comparative Examples 1 to 5 and Examples 1 to 5 are shown in Table 1 . Components mentioned in the upper part of Table 1 (under “Extrudate”) were mixed together and melt-extruded, before being pulverized to a dry powder to give a Dv90 between 50 and 100 pm. The extruded powder components were then dry blended with additives set out in the lower part of Table 1 .

[0094] Examples 6 to 9 are based on the extrudate portion of Example 1 , but contain higher fines levels of 10.1 and 13.3 wt% respectively, and only half the quantity of the dry blended additives (i.e. 0.5 wt% in total).

[0095] Comparative Examples 6 to 11 are also all based on the extrudate portion of Example 1 , but with different amounts and / or types of dry blended components. Formulations of Examples 6 to 9 and Comparative Examples 6 to 11 are set out in Table 2.

[0096] Table 1 - Powder coating compositions (wt%) [1]

[0097] 1] C.Ex. = comparative example. Ex. = example

[0098] [2] Caboxyl-functional unsaturated polyester resin

[0099] [3] Bisphenol A epoxy resin [4] Total amount of pigment and filler

[0100] [5] Total amount of flow aid, wax, antioxidant, benzoin and viscosity modifier

[0101] [6] Syloid™ C812 silica, D[4,3] average particle size of 11 pm, pore volume 2.0 mL / g.

[0102] [7] Weight% of particles with a particle size of <10 pm (based on total powder coating composition) Table 2 - Formulations of Examples 6-9 and Comparative Examples 6-11 (wt parts)

[0103] 1] The extrudate of Example 1

[0104] [2] Syloid™ C812 silica, D[4,3] average particle size of 11 pm, pore volume 2.0 mL / g.

[0105] [3] Aerosil™ R972 fumed (non-porous) silica

[0106] [4] HDK™ H3004 fumed (non-porous) hydrophobic silica

[0107] [Experiment 1 - Comparison of deposited fines]

[0108] 400g powder coating was added to a 3L fluid bed. Powder was sprayed using a GEMA negative corona gun at a set voltage of 90 kV (or 70 kV for Comparative Example 6 and Examples 8 and 9) onto a fixed 15 cm x 10 cm aluminium panel from a distance of 40 cm. A total of 1-2 g powder coating was deposited on the substrate.

[0109] Powder deposited on the panel was sampled and particle size measurements made using a Mastersizer™ Coulter Counter. This was compared to a sample of pre-sprayed powder collected from the fluid bed.

[0110] Table 3 shows the relative proportion of fines (in terms of number%) in the deposited layer compared to the unsprayed material. Table 3 - Proportion (number%) fines in deposited layer compared to pre-sprayed composition

[0111] The deposited coatings of the Examples incorporate a substantially higher proportion of fines compared to the Comparative Examples.

[0112] Figures 1 and 2 show plots of the number of particles in the deposited and pre-sprayed powder coating composition over a wider particle size range for Comparative Example 3 and Example 3 respectively. The particles of Example 3 are more evenly incorporated into the powder coating layer across all particle size ranges compared to Comparative Example 3.

[0113] Similar effects are observed for Comparative Example 6 and Examples 7 and 8 in Figures 3 to 5 respectively. The Examples have more consistent deposition profiles compared to the porous silica-free comparative example.

[0114] [Experiment 2 - Coating layer integrity]

[0115] Powder coatings were deposited onto panels in the same way as described above for Experiment 1 , except that the panels used were 30 x 15cm. The powder coating layer was then cured in a convection oven at 190 °C for 15 minutes. Panels of varying powder weight giving rise to varying coating thicknesses were prepared. Reflectance of each sample was taken from the centre of the panel and measured using a Datacolour 800 instrument. Results are shown in Table 4. Table 4 - Optical characteristics of powder coating films

[0116] D65 AE* signifies the extent of colour change of the sample compared to the thickest powder coating film. The AL* value represents the difference in transmission / reflectance.

[0117] These results show that the Example maintained its optical characteristics to a much greater extent at lower thicknesses compared to the Comparative Example, in that the Example exhibited smaller differences in colour change and transmission. This is also illustrated in Figures 6 and 7, which highlight that the opacity over visible wavelengths (450 to 700 nm) in Example 3 is maintained better at lower coating thicknesses compared to Comparative Example 3. These results are indicative of improved coating film evenness and lay-down at low film thicknesses in the Example compared to the Comparative Example.

[0118] [Experiment 3 - Gloss and Distinctness of Image]

[0119] Powder coatings were spray applied to 15 x 30 cm aluminium panels in a similar way to that described in Experiment 1 , and the samples were then cured in a convection oven. Curing conditions were 180 °C for 15 minutes for Comparative Examples 3 & 4 and for Examples 3 & 4. For Examples 1 , 6, and 8-9 and for Comparative Examples 1 & 6-11 , curing was carried out at 190°C for 15 minutes. 60° gloss and distinctness of image (DOI) values for the cured coatings were then obtained at 10 different points of the surface using a Rhopoint IQ meter. DOI measurements were recorded using the same instrument as for the gloss measurements and were also taken at an angle of 60°. Readings were taken at 10 different points of the surface. Averaged 60° gloss and DOI results are listed in Table 5.

[0120] Table 5 - Gloss and DOI measurements

[0121] 1] The powder coating had very poor fluidity [2] The powder had extremely poor deposition efficiency and did not cure. Its properties were not measured The results show that, at relatively thin film thicknesses of less than 50 pm, the Examples generally have both high 60° gloss values and high 60° DOI values, whereas the Comparative Examples generally have worse DOI values and in most cases also worse gloss. The results also show that the Examples can exhibit comparable or even improved gloss and DOI values at thinner film thicknesses compared to Comparative Examples at higher film thicknesses. Even where values between Comparative Examples and Examples at comparable film thicknesses are similar, the Examples exhibit improved fines utilization.

[0122] [Experiment 4 - Spray application performance]

[0123] Powder coatings of Example 5 and Comparative Example 5 were spray-applied to the convex surface of a curved aluminium sheet substrate. The depth of the corresponding concave surface was 50 mm. Photographs of the resulting covered substrates (after spraying, but before curing) are shown in Figures 8 and 9 for Comparative Example 5 and Example 5 respectively, where Figures 8a and 9a show the convex surfaces, 1 , and Figures 8b and 9b show the concave surfaces, 2.

[0124] Coverage on the convex surfaces, 1 , was better for Example 5 compared to Comparative Example 5. Coating coverage of the concave surface, 2, showed a starker improvement for Example 5 compared to Comparative Example 5, particularly at the apex of the curved surface.

[0125] Figures 10 and 11 show the results of a similar test using, respectively, Comparative Example 5 and Example 5, on a steel substrate having recesses of various depths, where spray application was directed towards the recessed face of the substrate.

[0126] The substrate had flat plate edges, 3, and recesses of different depths as follows: 4 = 44 mm, 5 = 20 mm, 6 = 35 mm, and 7 = 46 mm.

[0127] Figures 12 and 13 show the side edges, 8, of same substrates as shown in Figures 10 and 11 (for Comparative Example 5 and Example 5 respectively).

[0128] The powder coating of Example 5 in each case shows more uniform distribution, particularly in the recessed areas. Example 5 also shows better “wrap-around” performance, with better coverage of surfaces, 8, that are not directly targeted by the sprayed coating. Example 5 also exhibited improved evenness of coverage across the flat plate edges, 3, and the recessed areas, 4-7.

[0129] Comparative Example 5, by contrast, not only had poorer “wrap-around” performance onto the side surfaces, 8, but the coverage overall was also substantially less even, with comparatively thicker coating at the flat plate edges, 3, of the recessed substrate, and relatively poor penetration and evenness of coverage in the recessed areas, 4-7.

[0130] After curing at 200 °C for 15 minutes, poor coating integrity was observed for Comparative Example 5 in the concave region of the curved aluminium substrate, 2, and also in regions 4 and 7 of the recessed substrate. Conversely, coating integrity was good in all areas of the curved and recessed substrates for Example 5.

Claims

Claims1 . A sprayable powder coating composition comprising a first powder coating component dry blended with a silica, in which the first powder coating component comprises a curable resin and a curing additive for curing the curable resin and has a Dv90 particle size of 150 pm or less, at least 95 wt% of the dry blended silica is porous silica with a D[4,3] average particle size in the range of from 2 to 20 pm and a pore volume of in the range of from 0.8 to 3.0 mL / g, and the amount of the dry blended porous silica in the powder coating composition is in the range of from 0.01 to 0.70 wt%.

2. The powder coating composition as claimed in claim 1 , comprising at least 5 wt% of particles with a particle size of 10 pm or less.

3. The powder coating composition as claimed in claim 1 or claim 2, wherein at least 95 wt% of all silica in the coating composition is the dry blended porous silica with an average particle size of 2 to 20 pm.

4. The powder coating composition as claimed in any one of claims 1 to 3, wherein the curable resin is selected from carboxyl-functional polyesters, carboxyl-functional polyacrylates, hydroxy-functional polyesters and hydroxy-functional polyacrylates, and the curing additive is selected from p-hydroxyalkylamides, polyisocyanates and epoxyfunctional resins.

5. The powder coating composition as claimed in any one of claims 1 to 4, in which there are one or more additional powder coating components each comprising a curable resin and a curing additive for curing the curable resin, wherein the curable resin and / or curing additive in each additional powder coating component is different from that of the first powder coating component, and / or the relative ratio of curable resin to curing additive in each additional powder coating is different from that of the first powder coating component.

6. The powder coating composition as claimed in any one of claims 1 to 5, wherein the total amount of the first powder coating component and any additional powder coating components in the powder coating composition is at least 50 wt%.

7. The powder coating composition as claimed in any one of claims 1 to 6 additionally comprising one or more other dry-blended additives.

8. The powder coating composition as claimed in claim 7, wherein the other dry- blended additives comprise dry blended inorganic particulate additives, the amount of dry-blended inorganic particulate additives in the powder coating composition being up to 35 wt%.

9. The powder coating composition as claimed in any one of claims 1 to 8, wherein one or more of the following characteristics apply:(i) the porous silica has a bulk density of 0.8 g / cm3or less;(ii) the amount of particles with a particle size of 10 pm or less is at least 7 wt%, at least 10 wt% or at least 12 wt%;(iii) the total amount of dry blended silica, alumina and aluminium hydroxide is in the range of from 0.1 to 1.5 wt%.

10. The powder coating composition as claimed in any one of claims 1 to 9, wherein the dry-blended silica comprises no fumed silica.

11. A substrate coated with a powder coating composition according to any one of claims 1 to 10, optionally after curing.

12. The substrate as claimed in claim 11, where the powder coating composition is cured and the cured powder coating composition has one or more of the following characteristics:(i) a film thickness of 50 pm or less;(ii) a 60° gloss value of 85 or more;(iii) a 60° distinctness of image value of 30 or more.

13. A method of coating a substrate comprising spray-applying a powder coating composition according to any one of claims 1 to 10 to a surface of the substrate and curing the powder coating composition.

14. The method as claimed in claim 13, wherein at least 85% of particles (on a numerical basis) of the powder coating composition having a particle size of 10 pm or less are deposited on the substrate.

15. The method as claimed in claim 13 or claim 14, wherein the cured powder coating composition has one or more of the following characteristics:(i) a film thickness of 50 pm or less;(ii) a 60° gloss value of 85 or more;(iii) a 60° distinctness of image value of 30 or more.

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